1. Field of the Invention
This invention relates to the synthesis of phthalonitrile monomers and oligomeric resins and, more specifically, to phosphazene-containing amines used as curing agents for the polymerization of phthalonitrile monomers and oligomeric resins and a method therefor.
2. Description of the Prior Art
Interest in fiber-reinforced composites for advanced aerospace applications has led to the search for high temperature polymers that are easily processed and exhibit high thermal and oxidative stability. Epoxies and polyimides are now being used but each has its disadvantages. Conventional epoxy-based composites and adhesives have a 200.degree. C. maximum service limit and polyimide resins used in composites matrices have a 300.degree. C. maximum service limit. Advanced design concepts, especially in the aerospace industry, demand even higher temperature requirements for polymeric materials.
A major problem of the polyimide system is the inability to process void- and blister-free components in high yield because of the evolution of volatile components formed during the polymerization condensation reaction. Other problems associated with both polyimides and epoxies include their brittleness, water absorptivity and engineering reliability.
Phthalonitrile resins, such as diether-linked phthalonitrile resins, are proving to be superior in physical and chemical properties to epoxies, polyimides and other plastics as matrices for fiber-reinforced composites. A major advantage of phthalonitrile resins, compared to other plastics, is their ability to withstand temperatures in excess of 300.degree. C. for extended periods without permanent damage to the coatings, plastics or composites made therefrom. Such resins usually contain a substantial proportion of aromatic structures, but cured polymers composed solely of aromatic rings tend to be brittle and intractable. A resin having flexible linkages between the aromatic rings minimizes or greatly reduces brittleness and intractability. Phthalonitrile resins with diether linkages are materials which meet these goals. Examples of these phthalonitrile-based polymers are found in U.S. Pat. Nos. 4,223,123; 4,226,801; 4,234,712; and 4,238,601 issued to T. M. Keller and J. R. Griffith, all of which are wholly incorporated herein by reference. However, polymerization and cure of these monomers in the neat state generally requires heating for several days at extremely high temperatures.
U.S. Pat. No. 4,408,035, wholly incorporated herein by reference, teaches curing of phthalonitrile monomers with a nucleophilic aromatic amine. The monomer, 4,4'-bis(3,4-dicyanophenoxy)biphenyl, has a melting point of 232.degree.-234.degree. C. The aromatic diamines covered in the above patent are somewhat volatile at the required processing melt temperature and potentially can cause void problems when used in an amount greater than 5% by weight. This is especially true if the amine is not dried before using as a curing agent. Thus, it is advantageous for a resin not to produce gaseous products when cured. Also, the chemical makeup of the polymer must be such that it consists of units having known resistance to bond-rupture under thermal, oxidative and hydrolytic conditions.
Tentative U.S. Pat. No. 5,003,039, to be issued Mar. 26, 1991, to Teddy Keller, based upon U.S. Ser. No. 07/273,430, and wholly incorporated herein by reference, teaches 1,3-bis(3-aminophenoxy)benzene as a curing agent for a rapid synthesis of phthalonitrile resins. This curing agent affords void-free components, but requires an initial processing temperature below 240.degree. C.
Phosphazenes are presently the most important and most thermally characterized of the phosphorous-containing inorganic polymers. Many such polymers with a variety of substituents at the phosphorus have been prepared. They often exhibit useful properties including fire retardancy, low temperature flexibility, and resistance to chemical attack.
Linear polyphosphazenes are commonly synthesized at elevated temperatures involving ring-opening polymerization of cyclic trimers such as hexa-substituted cyclotriphosphazene. The intermediate leading to ring openings of the hexa(amine-substituted cyclotriphosphazene) or the linear poly(amine-substituted phosphazene), if the process occurs during the thermal cure, would be expected to react with the phthalonitrile-based resin systems.